Synthesis of Protected 3&#39;-Amino Nucleoside Monomers

ABSTRACT

Orthogonally protected 3′-amino nucleoside monomers and efficient methods for their synthesis are described. The methods employ selective protection of the 3′-amino group in the presence of the unprotected nucleoside base.

This application claims priority to U.S. provisional application Ser.No. 60/585,193, filed on Jul. 2, 2004, which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to efficient methods of synthesis oforthogonally protected 3′-amino nucleoside monomers, useful for thesynthetic preparation of oligonucleotide analogs, and to theorthogonally protected monomers prepared by such methods.

REFERENCES

-   Asai, A. et al., Cancer Research 63(14):3931-9 (2003).-   Carpino, L. A. et al., J. Org. Chem. 45:4250-2 (1980).-   Carpino, L. A. et al., J. Org. Chem. 54:5887-97 (1989).-   Cech, D. et al., Coll. Czech. Chem. Comm. 61:S297-S300 (1996).-   Chen, J.-K., Schultz, R. G., Lloyd, D. H. and Gryaznov, S. M.,    Nucleic Acids Res. 23: 2661-2668 (1994).-   Escude, C., Giovannageli, C., Sun, J.-S., Lloyd, D.-H., Chen, J.-K.,    Gryaznov, S. M., Garestier, T. and Helene, C., Proc. Natl. Acad.    Sci. USA 93:4365-4369 (1996).-   Giovannangeli, C., Diviacco, S., Labrousse, V., Gryaznov, S. M.,    Charneau, P. and Helene, C., Proc. Natl. Acad. Sci. USA 94:79-84    (1997).-   Gryaznov, S. M. and Chen, J.-K. J. Am. Chem. Soc. 116:3143-3144    (1994).-   Gryaznov, S. M. et al., Nucleosides, Nucleotides & Nucleic Acids    22(5-8):577-581 (2003).-   Gryaznov, S. M., Lloyd, D. H., Chen, J.-K., Schultz, R. G.,    DeDionisio, L. A., Ratmeyer, L. and Wilson, W. D., Proc. Natl. Acad.    Sci. USA 92:5798-5802 (1995).-   Gryaznov, S. M., Pongracz, K., and Matray, T., PCT Pubn. No. WO    2001/018015 (Mar. 15, 2001).-   Gryaznov, S. M., Skorski, T., Cucco, C., Nieborowska-Skorska, M,    Chiu, C. Y., Lloyd, D. H., Chen, J. K., Koziolkiewicz, M. and    Calabretta, B. Nucleic Acids Res. 24:1508-1514 (1996).-   Nelson, J. S. et al., J. Org. Chem. 62:7278-7287 (1997).-   Pongracz, K. and Gryaznov, S. M., Tetrahedron Letters 40(43):    7661-7664 (1999).-   Skorski, T., Perrotti, D., Nieborowska-Skorska, M., Gryaznov, S. M.    and Calabretta, B., Proc. Natl. Acad. Sci. USA 94:3966-3971 (1997).-   Vincente, S. et al., J. Org. Chem. 64:991-997 (1999).-   Vu, H. et al., Tetrahedron Letters 31(5):7269-7272 (1990).-   Wang, E. S. et al., Blood 103(1):258-266 (2004).-   Zaitseva, G. V. et al., Nucleosides & Nucleotides 13(1-3):819-838    (1994).-   Zaitseva, V. E. et al., Sov. J. Bioorg. Chem. 10(5)5:369-378    (transl. from Bioorg. Khim. 10(5):670-680) (1984).

BACKGROUND OF THE INVENTION

The use of oligonucleotides and oligonucleotide analogs as therapeuticagents, based on specific binding to target nucleic acid sequences or toproteins, has been extensively researched. Structurally modifiedoligonucleotide analogs have been designed which lack the nucleasesusceptibility of natural (phosphodiester-linked) oligonucleotides andwhich, in some cases, exhibit other beneficial properties such asenhanced binding to targets or enhanced specificity of binding. One suchclass of oligonucleotide analog is the N3′→P5′ phosphorodiamidate-linkedoligonucleotide (Gryaznov and Chen, 1994; Chen et al., 1994). Thesecompounds are nuclease resistant, form stable duplexes withcomplementary RNA and duplex DNA targets, and have demonstratedsignificant sequence-specific antisense activity both in vitro and invivo (Gryaznov et al., 1995; Escude et al., 1996; Gryaznov et al., 1996;Giovannangeli et al., 1997; Skorski et al., 1997). The related N3′→P5′thiophosphoramidate oligonucleotides retain the high RNA bindingaffinity of N3′→P5′ phosphoramidates and also exhibit improved acidstability (Pongracz and Gryaznov, 1999; Gryaznov et al., 2001). CertainN3′→P5′ thiophosphoramidate oligonucleotides have shown therapeuticallypromising telomerase inhibiting activity (Gryaznov et al., 2003; Asai etal., 2003; Wang et al., 2004).

Stepwise, sequence-controlled preparation of N3′→P5′ phosphoramidate orthiophosphoramidate oligonucleotides employs 3′-amino nucleosidemonomers in which the 3′-amino group is protected during addition, thendeprotected for addition of a further monomer to the growingoligonucleotide chain (see e.g. Gryaznov and Chen, 1994; Pongracz andGryaznov, 1999). Because the groups on the nucleoside bases which aretypically protected during synthesis are primary amino groups, the needfor protection of the 3′-amino group in the presence of these groups hascomplicated the preparation of these monomers. Existing procedures (seee.g. Nelson et al., 1997) entail multiple steps of protection andgenerally involve conversion of a 3′-hydroxyl to a 3′-azido (—N₃) group,which is later reduced to the 3′-amine. These procedures are timeconsuming, expensive, and result in low overall yields of the monomers.Accordingly, improving the efficiency of these syntheses is desired, andwill facilitate the preparation of N3′→P5′ phosphoramidate orthiophosphoramidate oligonucleotides.

SUMMARY OF THE INVENTION

The present invention provides, in one aspect, a method of preparing anadenosine, guanosine or cytosine monomer having a protected nucleosidebase and a protected 3′-amino group, wherein the base and the 3′-aminogroup are orthogonally protected. In one embodiment, the methodcomprises the steps of:

(a) providing a 3′-amino-3′-deoxy adenosine, cytosine, or guanosinemonomer in which the 5′-hydroxyl group, nucleoside base, and 3′-aminogroup are unprotected;

(b) selectively reacting the 3′-amino group with a first protectinggroup;

reacting the 5′-hydroxyl group with a second protecting group; and

reacting the nucleoside base with a third protecting group;

wherein the first protecting group can be removed from the 3′-aminogroup under conditions which do not deprotect the nucleoside base, andthe second protecting group can be removed from the 5′-hydroxyl groupunder conditions which do not deprotect the nucleoside base or the3′-amino group. The method may further comprise the step of (c) removingthe second protecting group from the 5′-hydroxyl group, under conditionswhich do not deprotect the nucleoside base or the 3′-amino group.

The first protecting group may be acid labile; e.g. a triarylmethylgroup, such as triphenylmethyl (trityl), monomethoxytrityl (MMT), ordimethoxytrityl (DMT). In another embodiment, the first protecting groupis fluorenylmethoxycarbonyl (Fmoc) or a derivative thereof, removablewith a basic amine such as DBU or piperidine.

In one embodiment, preferably employed for an adenosine or cytosinemonomer, the second and third protecting groups are both acyl groups,e.g. benzoyl groups, and the conditions of (c) comprise mild treatmentwith hydroxide ion.

In another embodiment, the second protecting group is a base labiletrialkyl silyl ether, e.g. a trimethylsilyl (TMS) ether, the thirdprotecting group is an acyl group, and the conditions of (c) comprisemild treatment with hydroxide ion.

In a further embodiment, the second protecting group is a fluoridelabile silyl ether, the third protecting group is an acyl group, and theconditions of (c) comprise treatment with fluoride ion. Such fluoridelabile silyl ethers include, for example, a tert-butyldimethyl silylether, a tert-butyldiphenyl silyl ether, a diphenylmethyl silyl ether,and a tri(isopropyl) silyl ether. In one embodiment, the fluoride labilesilyl ether is a tert-butyldimethyl silyl (TBDMS) ether.

In an alternative embodiment, the method comprises the steps of:

(a) providing a 3′-amino-3′-deoxy adenosine, cytosine, or guanosinemonomer in which the 5′-hydroxyl group, nucleoside base, and 3′-aminogroup are unprotected;

(b) selectively reacting the 3′-amino group with a first protectinggroup; and

selectively reacting the nucleoside base with a further protectinggroup;

wherein the first protecting group can be removed from the 3′-aminogroup under conditions which do not deprotect the nucleoside base.

In this embodiment of the method, the 5′-OH group remains substantiallyunreacted under conditions of protection of the nucleoside base, anddoes not require separate protection. In one embodiment, the monomer isa cytosine monomer, the first protecting group is as described above,and the further protecting group is an acyl group, preferably a benzoylgroup. The acyl protecting group is preferably incorporated by means ofreaction with an acyl anhydride.

In other embodiments, the monomer is a guanosine or adenosine monomer,the first protecting group is as described above, and the furtherprotecting group is a formamidinyl group, such as a dialkylformamidinylgroup.

In a related aspect, the invention provides a method of preparing athymidine monomer having a free 5′-hydroxyl group and a protected3′-amino group, the method comprising:

(a) providing a 3′-amino-3′-deoxy thymidine monomer in which the5′-hydroxyl group and 3′-amino group are unprotected; and

(b) selectively reacting the 3′-amino group with a first protectinggroup.

The first protecting group may be acid labile; e.g. a triarylmethylgroup, such as triphenylmethyl (trityl). In another embodiment, thefirst protecting group is fluorenylmethoxycarbonyl (Fmoc) or aderivative thereof, removable with a basic amine such as DBU orpiperidine.

The starting and product monomers in these synthetic methods preferablycomprise a 2′ group selected from hydrogen, hydroxy, lower alkoxy, loweralkyl, and fluoro. In selected embodiments, the monomer comprises a 2′group selected from hydrogen, hydroxy, methoxy, and fluoro. In oneembodiment, the monomer is a 2′,3′-dideoxy monomer, such that the 2′group is hydrogen.

In another aspect, the invention provides an adenosine, guanosine orcytosine monomer having a protected 3′-amino group and a nucleoside basewhich is (i) unprotected or (ii) protected such that the protected3′-amino group can be deprotected under conditions which do notdeprotect the nucleoside base. In selected embodiments, the 3′-aminogroup is protected with an acid labile protecting group; e.g. atriarylmethyl group, such as triphenylmethyl (trityl). In otherembodiments, the 3′-amino group is protected withfluorenylmethoxycarbonyl (Fmoc) or a derivative thereof which isremovable with a basic amine such as DBU or piperidine.

In one embodiment, the nucleoside base is unprotected; in otherembodiments, the nucleoside base is protected with an acyl group.Preferably, the acyl group is benzoyl when the monomer is an adenosineor cytosine monomer and isobutyryl when the monomer is a guanosinemonomer. In still further embodiments, the nucleoside base is protectedwith a formamidinyl group, preferably a nucleoside base is protectedwith a dialkylformamidinyl group.

The 5′-hydroxyl group of the monomer may be unprotected, or it may beprotected such that it can be deprotected under conditions which do notdeprotect the nucleoside base or the 3′-amino group. For example, it maybe protected with a silyl ether which is labile to mild base of tofluoride ion, such as those noted above.

The monomer preferably comprises a 2′ group selected from hydrogen,hydroxy, lower alkoxy, lower alkyl, and fluoro. In selected embodiments,the monomer comprises a 2′ group selected from hydrogen, hydroxy,methoxy, and fluoro. In one embodiment, the monomer is a 2′,3′-dideoxymonomer, such that the 2′ group is hydrogen.

These and other objects and features of the invention will become morefully apparent when the following detailed description of the inventionis read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scheme showing alternative exemplary methods for synthesisof a 3′-amino adenosine monomer, also applicable for synthesis of3′-amino cytosine monomers, in accordance with various embodiments ofthe invention;

FIG. 2A shows an exemplary method for synthesis of a 3′-amino guanosinemonomer, in accordance with one embodiment of the invention;

FIG. 2B shows an alternative exemplary method for synthesis of a3′-amino guanosine monomer, in accordance with one embodiment of theinvention;

FIG. 2C shows a method for synthesis of 3′-amino cytosine monomer, inaccordance with a further embodiment of the invention; and

FIG. 3 shows an exemplary method for synthesis of a 3′-amino thymidinemonomer, in accordance with a further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION I. Definitions

The terms below have the following meanings unless indicated otherwise.

“Orthogonally protected”, with respect to a plurality of protectedfunctional groups in the same molecule, indicates that it is possible todeprotect any selected member of the group without deprotecting theother group(s).

“Selectively protected”, with respect to protection of a targetfunctional group in a molecule having a plurality of unprotectedfunctional groups, indicates that, in a reaction of the molecule with aprotecting reagent, the target functional group is protected to agreater extent than any non-target functional group. The extent ofreaction of the target functional group relative to that of anynon-target functional group(s) is greater than 1:1, preferably greaterthan 2:1, and more preferably greater than 3:1 or higher, e.g. greaterthan 9:1. The same definition applies to the terminology “selectivelyreacting” a given functional group with a protecting group.

“Reacting with a protecting group”, as used herein, is equivalent to“providing with a protecting group”, “protecting with a protectinggroup”, or “reacting with a protecting group reagent”.

“Alkyl” refers to a fully saturated acyclic moiety consisting of carbonand hydrogen, which may be linear or branched. Examples of alkyl groupsare methyl, ethyl, n-butyl, t-butyl, n-heptyl, and isopropyl. Generallypreferred are lower alkyl groups, having one to six carbon atoms, asexemplified by methyl, ethyl, n-butyl, i-butyl, t-butyl, isoamyl,n-pentyl, and isopentyl. In other embodiments, lower alkyl includesgroups having one to four carbon atoms, or 1-2 carbon atoms (methyl andethyl).

“Cycloalkyl” refers to a saturated cyclic hydrocarbon, preferably having4 to 7 carbon atoms, more preferably 5 or 6 (i.e. cyclopentyl orcyclohexyl).

“Alkenyl” refers to an unsaturated acyclic moiety consisting of carbonand hydrogen, which may be linear or branched, having one or more doublebonds. Generally preferred are lower alkenyl groups, having two to six,or two to four, carbon atoms. “Alkynyl” refers to an unsaturated acyclicmoiety consisting of carbon and hydrogen, which may be linear orbranched, containing one or more triple bonds. Generally preferred arelower alkynyl groups, having two to six, or two to four, carbon atoms.

“Aryl” refers to a substituted or unsubstituted monovalent aromaticradical, generally having a single ring (e.g., benzene) or two condensedrings (e.g., naphthyl), where monocyclic aryl groups are preferred. Theterm includes heteroaryl groups, which are aromatic ring groups havingone or more nitrogen, oxygen, or sulfur atoms in the ring, such asfuryl, pyrrole, pyridyl, and indole. By “substituted” is meant that oneor more ring hydrogens in the aryl group, preferably one or two ringhydrogens, is replaced with a group preferably selected from fluorine,chlorine, bromine, methyl, ethyl, methoxy, halomethoxy, and halomethyl.Preferred aryl groups for use in protecting groups are carbocyclic arylswhich are unsubstituted or substituted with lower alkoxy (in addition tothe substituent linking the group to the protected moiety).

“Aralkyl” refers to an alkyl, preferably lower (C₁-C₄, more preferablyC₁-C₂) alkyl, substituent which is further substituted with an arylgroup, preferably a monocyclic aryl group; examples are benzyl(—CH₂C₆H₅) and phenethyl.

“Acyl” refers to a substituent of the form R(C═O)—, where R is alkyl,alkenyl, alkynyl, aralkyl, or aryl as defined above, and is preferablyselected from lower alkyl and monocyclic carbocyclic aryl. Examplesinclude benzoyl (Ph(C═O)—), acetyl (CH₃(C═O)—) and isobutyryl((CH₃)₂CH(C═O)—).

II. Synthesis of Protected Monomers

The invention provides, in one aspect, efficient syntheses oforthogonally protected 3′-amino-5′-hydroxyl nucleoside monomers, inwhich the amino groups on the 3′-position and the nucleoside base areorthogonally protected, starting from the corresponding unprotected3′-amino nucleosides. Typically, the product monomers are thenphosphitylated at the free 5′-hydroxyl for use in synthesis of N3′→P5′phosphoramidate or thiophosphoramidate oligonucleotides. (See e.g. FIG.2B and FIG. 3.)

A. Starting Materials

The starting material for the monomer synthesis is a 3′-amino nucleosidehaving the general formula below:

where BASE is an unprotected nucleoside base selected from guanine (G),adenine (A), thymine (C) and cytosine (C). The 2′-substituent X may behydrogen, as in 2′,3′-dideoxy monomers, for producing DNA analogs.Alternatively, X may be hydroxy or lower alkoxy, such as methoxy, forproducing RNA or O-alkyl RNA analogs. The 2′-substituent may also befluorine (see e.g. U.S. Pat. No. 5,684,143) or lower alkyl, such asmethyl.

The 3′-amino-2′,3′-dideoxy nucleosides, useful as starting materials inthe syntheses herein, can be obtained commercially from Metkinen Oy,located in Littoinen, Finland. The 3′-amino-2′,3′-dideoxy thymidinestarting material is also commercially available from various sourcessuch as Dalton Chemical Labs (Toronto, Canada) and MP Biomedicals Inc.(Irvine, Calif.). Various synthetic preparations of the3′-amino-2′,3′-dideoxy nucleosides have been reported in the literature(e.g. Zaitseva et al., 1994; Cech et al., 1996; Zaitseva et al., 1984).In accordance with Zaitseva et al., 1994, 3′-amino-2′,3′-dideoxythymidine can be converted into the corresponding adenosine or guanosinemonomers via enzymatic transglycosylation.

B1. Protection Strategies: Overview

In accordance with the invention, efficient procedures are provided forconverting the above-depicted nucleosides into the correspondingbase-protected (in the case of A, G, and C), 3′-amino-protectedmonomers. Because the groups on the bases which must be protected arealso amino groups, selective protection of the 3′-amino group in thepresence of the unprotected base, or vice versa, has not been carriedout successfully in the past, and existing procedures for thepreparation of these monomers (see e.g. Nelson et al., 1997) entailmultiple steps of protection and generally involve conversion of a3′-hydroxyl to an azido group, which is later reduced to the 3′-amine.

The present inventors have provided efficient methods for selectivelyprotecting the 3′-amino group in the presence of the nucleoside bases,such that the nucleoside base can then be protected with an orthogonalprotecting group; that is, one that is not removed under selectedconditions that are effective to remove the 3′-amino protecting group(or the 5′-hydroxyl protecting group, if present).

In addition, the 3′-amino group is also selectively protected in thepresence of the 5′-hydroxyl group, or vice versa. Accordingly, it ispossible to provide the free 5′-hydroxyl with a protecting group, whichcan be removed under selected conditions which do not remove either the3′-amino protecting group or the base protecting group. (Alternatively,as in the case of thymidine, the free 5′-hydroxyl can be directlyphosphitylated.)

The protecting group for the 5′-hydroxyl may be, for example, a silylgroup, which can be removed under conditions, such as mild base orfluoride ion, which do not remove the protecting groups (typically acylor amidinyl groups) used for the nucleotide base. Alternatively, thesame protecting group (such as an acyl group) can be used for the5′-hydroxyl and nucleotide base, with conditions employed for itsremoval (e.g. mild base) that will deprotect the hydroxyl group but notthe nucleotide base amino groups.

In a further strategy, the nucleotide base can be protected underconditions that leave the 5′-hydroxyl unreacted, and thus not requiringprotection.

In all cases, the protecting groups employed for the nucleotide basesare stable to the conditions of monomer coupling reactions employed inoligonucleotide synthesis. Methods for preparing N3′→P5′ phosphoramidateor thiophosphoramidate oligonucleotides from the monomers describedherein are described, for example, in Gryaznov and Chen (1994) andPongracz and Gryaznov (1999).

B2. Protection Strategies: A, G, and C Monomers

The invention provides, for adenosine, guanosine or cytosine monomers,methods of preparing said monomers having a free 5′-hydroxyl group, aprotected nucleoside base, and a protected 3′-amino group, where thebase and 3′-amino group are orthogonally protected, as defined above.The starting material is a 3′-amino-3′-deoxy adenosine, cytosine, orguanosine monomer in which the 5′-hydroxyl group, nucleoside base, and3′-amino group are unprotected.

In one general strategy, the method comprises selectively reacting the3′-amino group with a first protecting group, reacting the 5′-hydroxylgroup with a second protecting group, and reacting the nucleoside basewith a third protecting group. The first protecting group (for the3′-amino) is such that it can be removed from the 3′-amino group underconditions which do not deprotect the nucleoside base. Moreover, thesecond protecting group (for the 5′-hydroxyl), when present, is suchthat it can be removed from the 5′-hydroxyl group under conditions whichdo not deprotect the nucleoside base or the 3′-amino group.

After the desired reactive groups are protected, in accordance with theabove general strategy, the second protecting group, if present, isremoved from the 5′-hydroxyl group, under conditions which do notdeprotect the nucleoside base or the 3′-amino group. The monomer canthen be phosphitylated, e.g. for use in oligonucleotide synthesis.

Typically, the first, second, and third protecting groups are applied inthat order, although the second and third groups may in fact be thesame, and thus applied in a single reaction. Moreover, protection of the5′-hydroxyl group, in strategies employing this step, can occur beforeor after the protection of the 3′-amino group. The two reactions aretypically carried out in a one-pot reaction, although the reagents arepreferably not added simultaneously. Preferably, the 3′-amino protectingreagent is added first, as illustrated in the examples below.

In selected embodiments, transient protection of the 5′-hydroxyl groupwith a base labile silyl ether (e.g. TMS) is employed during the baseprotection step, as shown, for example, in FIG. 1 (conversion ofstructure 2 to structure 6) and in FIG. 2B, as discussed further below.

In other embodiments, the 5′-hydroxyl group and nucleotide bases areprotected with the same reagent, followed by selective deprotection ofthe hydroxyl group. Such a reagent is typically an acylating reagent,e.g. an isobutyryl halide or benzoyl halide. An exemplary scheme of thistype is shown in the conversion of structure 2 to 3 in FIG. 1, followedby treatment with base under mild conditions to selectively deprotectthe hydroxyl group (conversion of 3 to 6).

As noted above, the first protecting group, for protecting the 3′-amine,is one that is stable under conditions that can remove the secondprotecting group (for the 5′-hydroxyl), when present, but is labileunder conditions that do not remove the third protecting group (for thenucleoside base). In one embodiment, the first protecting group is acidlabile; e.g. a triarylmethyl group such as trityl (triphenylmethyl),monomethoxytrityl (MMT), or dimethoxytrityl (DMT).

Another group useful as the first protecting group isfluorenylmethoxycarbonyl (Fmoc), which is removable via non-hydrolyticcleavage with a basic amine, such as DBU, morpholine, or piperidine. Ahydroxide base (e.g. NaOH) is not suitable for this purpose, since thisreagent will typically remove other protecting groups in the molecule aswell. Derivatives of Fmoc which are removable by a similar mechanism mayalso be used. Such derivatives include those in which the fluorenylgroup of Fmoc is substituted, typically at the 2 and/or 7 position, witha group, such as a lower alkyl group, which will not impact the cleavagemechanism, or with an electron withdrawing group, such as halogen, whichincreases the base lability of the protecting group (Carpino et al.,1980). Also included are the thioxanthene dioxide analogs described inCarpino et al., 1989.

FIG. 1 shows exemplary permutations of a synthetic strategy which can beused for preparation of the subject monomers. These methods areparticularly suitable for A (adenosine) and C (cytosine) nucleosidemonomers. As described below, the methods can also be used, preferablywith some modifications, for G (guanosine) monomers. In the strategiesexemplified in FIGS. 1 and 2A-B, protection of the 5′-hydroxy isemployed. FIG. 2C, discussed further below, illustrates a strategy inwhich 5′-hydroxy protection is not employed.

As noted above, a preferred first protecting group (for the 3′-amine) isan acid labile group, such as trityl, MMT, or DMT, or a group, such asFmoc, which is labile to a basic amine reagent such as DBU. In oneembodiment, preferred second and third protecting groups are both aracylgroups, e.g. benzoyl groups. Conditions which are effective to deprotectthe 5′-hydroxyl group under conditions which do not deprotect thenucleoside base or the 3′-amino group, in this case, preferably comprisemild base treatment, such as treatment with hydroxide, e.g. ammoniumhydroxide. Such treatment is effective to remove the acyl group from the5′-hydroxyl and to convert diacylated base to monoacylated base. Such asscheme is illustrated in FIG. 1, in the conversion of intermediate 3 toproduct 6.

In another embodiment, the second protecting group, for protection ofthe 5′-hydroxyl, is a base labile trialkyl silyl ether, preferably TMS,and the third protecting group (for protection of the nucleotide base)is an acyl group, such as an alkanoyl, preferably isobutyryl, or benzoylgroup. Again, conditions which are effective to deprotect the5′-hydroxyl group under conditions which do not deprotect the nucleosidebase or the 3′-amino group, in this case, preferably comprise mild basetreatment, e.g. treatment with hydroxide. Such treatment is effective tocleave a trimethylsilyl ether and to convert diacylated base, ifpresent, to monoacylated base. An example of this scheme is alsoillustrated in FIG. 1, in the conversion of intermediate 2 to product 6.

Another preferred type of nucleotide base protecting group, particularlyfor adenosine and guanosine monomers, is a formamidinyl protectinggroup, such as a dialkyl-, di(cycloalkyl)-, or di(aralkyl)-formamidinylgroup, where “alkyl” is preferably C₁-C₄ and “cycloalkyl” is preferablyC₅-C₆. Specific examples include dimethylformamidinyl anddibenzylformamidinyl. These protecting groups can generally be removedfrom the nucleotide bases (e.g. at the end of synthesis) under milderconditions than are used to remove benzoyl or isobutyryl protectinggroups. See, for example, Vu et al., 1990, Vincent et al., 1999, and/orU.S. Pat. No. 5,281,701. As described therein, reaction of a primaryamine with a dialkylformamide dimethyl acetal, for example, provides thedialkylformamidinyl-protected amine. Deprotection can generally beeffected by treatment with aqueous or alcoholic ammonium hydroxide atroom temperature to about 55° C.

In a further embodiment, the second protecting group (for protecting the5′-hydroxyl) is a fluoride-labile silyl ether, and the third protectinggroup (for the nucleoside base) is an acyl group, such as a benzoylgroup, or a formamidinyl group as described above. In this case,conditions which are effective to deprotect the 5′-hydroxyl group, underconditions which do not deprotect the nucleoside base or the 3′-aminogroup, preferably comprise treatment with fluoride ion, e.g.tetrabutylammonium fluoride (TBAF). Such treatment is effective tocleave the silyl ether and to convert diacylated base, if present, tomonoacylated base. An example of this scheme is also illustrated in FIG.1, in the conversion of intermediate 5 to product 6. Exemplaryfluoride-labile silyl ethers include, for example, tert-butyldimethylsilyl ether, tert-butyldiphenyl silyl ether, diphenylmethyl silyl ether,tri(isopropyl) silyl ether, and others known in the art.

The above methods can also be used, with some modifications relatingprimarily to solubility, for guanosine (G) monomers. In one embodiment,illustrated in FIG. 2A, the 5′-hydroxyl group of the starting monomer 7is protected with a lipophilic group, such as TBDMS, at an early stageof the process, to facilitate solubility in the conventional solventpyridine.

As noted above, a preferred first protecting group (for the 3′-amine) isan acid labile group, such as trityl, DMT, or MMT, or a group, such asFmoc, which is labile to a basic amine reagent such as DBU. A tritylgroup is employed in the embodiment of FIG. 2A.

Depending on reaction conditions, small quantities (e.g. about 5%) ofthe ditritylated compound 9 may be observed in addition to the desired3′-monotritylated compound 8. Accordingly, “selective protection” asused herein indicates that the molecule having the desired state ofprotection is formed to a greater extent, i.e. in a ratio greater than1:1, than molecules in which non-target functional groups are protected,either exclusively or in addition to the target functional group.Preferably, the ratio is greater than 2:1, more preferably greater than3:1, and most preferably greater than 9:1.

The second protecting group (for the 5′-hydroxyl) is, in thisembodiment, a fluoride-labile silyl ether, and more preferably a groupwhich promotes solubility of the intermediate. A TBDMS ether, as shownin FIG. 2A, was found to promote solubility in the solvent pyridine.

The third protecting group is preferably an acyl group, e.g. anisobutyryl group, or a formamidinyl group as described above. Conditionswhich are effective to deprotect the 5′-hydroxyl group, under conditionswhich do not deprotect the nucleoside base or the 3′-amino group,preferably comprise treatment with fluoride ion, e.g. tetrabutylammoniumfluoride (TBAF). Such treatment is effective to cleave the silyl etherin compound 10 without affecting isobutyryl or trityl groups, as shownin FIG. 2A.

In another embodiment, such as illustrated in FIG. 2B, a more polarsolvent, such as DMF, is used to solubilize the starting monomer 7, andthe reaction scheme is otherwise similar to the “transient protection”scheme (using a TMS ether as the 5′-hydroxyl protecting group) shown forthe adenosine monomer 1 in FIG. 1.

FIG. 2C illustrates an embodiment of a synthetic strategy, particularlyapplicable to cytidine monomers, in which 5′-hydroxyl protection is notemployed. In this reaction scheme, following selective protection of the3′-amino group of the monomer 12 with, for example, a trityl group, theexocyclic amino group of the base is reacted with an acyl anhydride,such as benzoyl anhydride. The solvent preferably contains an alcohol,which is believed to compete with and suppress reaction of the5′-hydroxyl group. Examples include methanol, ethanol, and mixtures ofthese solvents with, for example, acetonitrile, DMF, or pyridine. In thereaction using benzoyl anhydride in ethanol or in 9:1acetonitrile:methanol, as described in Example 4, little (<5%) of the5′-benzoylated product was observed. The predominant 5′-hydroxyl monomer14 can then be phosphitylated by standard methods.

In another embodiment, particularly applicable to adenosine or guanosinemonomers, following selective protection of the 3′-amino group of themonomer with, for example, a trityl group, the exocyclic amino group ofthe base is protected with a dialkylformamidinyl,di(cycloalkyl)formamidinyl, or dibenzylformamidinyl group, preferably adimethylformamidinyl group. The 5′-hydroxyl group, which is expected toremain substantially or fully unreacted during these steps, can then bephosphitylated by standard methods.

B3. Protection Strategies: T Monomer

In another aspect, the invention provides a method of preparing athymidine monomer having a free 5′-hydroxyl group and a protected3′-amino group. The synthetic strategy differs from that describedabove, in that the thymine base does not generally require protectionunder conditions of oligonucleotide synthesis. The starting material inthis case is a 3′-amino-3′-deoxy thymidine monomer in which the5′-hydroxyl group and 3′-amino group are unprotected, and the methodcomprises selectively reacting said 3′-amino group with a firstprotecting group, such that the 5′-hydroxyl group remains substantiallyunprotected.

As in the syntheses described above, a preferred protecting group forthe 3′-amine is an acid labile group, such as a triarylmethyl group, ora group, such as Fmoc, which is labile to a basic amine reagent such asDBU. A trityl group is employed in the embodiment of FIG. 3.

The 5′-hydroxyl group can then be phosphitylated, as described e.g. inExample 5 below Note that this phosphitylation process is alsoapplicable to any of the 5′-hydroxyl, 3′-amino-protected andbase-protected monomers described above.

EXAMPLES

The following examples illustrate but are not intended to limit theinvention. For example, reaction conditions, such as choice of solvent,catalyst (e.g. triethylamine or diisopropyl ethyl amine), reactiontimes, and reaction temperatures, can generally be varied from thoseexemplified below, according to the knowledge of one skilled in the art,using routine experimentation. In some cases, order of addition ofreagents could be varied. Suitable solvents for the majority of thesereactions, where not otherwise indicated, generally include polaraprotic solvents such as pyridine, DMF, acetonitrile, or mixturesthereof. Suitable temperatures are generally in the range of −10° C. toroom temperature to about 55° C.

Example 1 Synthesis of N⁶-benzoyl-3′-aminotrityl-2′,3′-dideoxyadenosinefrom 3′-amino-2′,3′-dideoxyadenosine A. Method 1: Perbenzoylation Route(as Illustrated in FIG. 1, 1→2→3→6)

The starting monomer, 3′-amino-2′,3′-dideoxyadenosine (1, 10 mmol), wascoevaporated with dry pyridine, then suspended in 100 ml dry pyridinecontaining 6 eq. triethylamine, and the mixture was heated to 50° C.with stirring. Trityl chloride (1.1 eq.) was added, and stirring wascontinued for two hours at 50° C. The obtained clear solution containingthe 3′-tritylated monomer 2 was cooled to 0° C., 5 eq. benzoyl chloridewas added dropwise, and the reaction mixture was stirred for one hourand then poured into 100 ml of cold 5% sodium bicarbonate. Theprecipitated yellow gum was extracted with ethyl acetate, and the ethylacetate solution evaporated. The resulting oil (3, having a benzoylgroup at the 5′-hydroxyl and two at N⁶ of the base) was dissolved in 50ml pyridine:methanol:water 65:35:5 v/v/v, the solution was cooled to 0°C., and 50 ml 2M sodium hydroxide was added. After 25 min stirring at 0°C., the reaction mixture was neutralized with pyridinium hydrochloride,and the volume of the mixture was reduced by evaporation. After dilutionwith ethyl acetate, the organic layer was separated and washed withsatd. sodium bicarbonate and evaporated in vacuo. The product waspurified by silica gel flash column chromatography using methylenechloride:methanol 95:5 v/v solvent system. The isolated yield of product(6), having a free 5′-hydroxyl and monobenzoylated base, was 2.0 g(33.6%)

B. Method 2: Transient Protection Route (as Illustrated in FIG. 1,1→2→6)

The starting monomer, 3′-amino-2′,3′-dideoxyadenosine (1, 10 mmol), wasconverted to the 3′-protected monomer 2 as described above, i.e. bytreatment with 1.1 eq. trityl chloride in dry pyridine containing 6 eq.triethylamine at 50° C.

The clear solution of 2 was cooled to 0° C., and 5 eq.chlorotrimethylsilane was added dropwise, followed by stirring for 30min, producing a 5′-OTMS intermediate. Benzoyl chloride (5 eq) was thenadded dropwise, followed by stirring for two hours at room temperature,producing a monobenzoyl and/or dibenzoyl base protected intermediate.

The reaction mixture was cooled to 0° C., and 20 ml cold water wasadded, followed by 20 min stirring and addition of 20 ml conc. ammoniumhydroxide. Stirring was continued for 30 min, and the solution wasconcentrated in vacuo. The oily residue was taken up in ethyl acetateand washed with satd. sodium bicarbonate, and the solution was dried andevaporated in vacuo. The product was purified as above by silica gelchromatography. Isolated yield of product (6) was 3.6 g (60.2%).

In subsequent procedures, DMF or DMF/pyridine, with added triethylamine,was used as solvent for the initial tritylation reaction, and theintermediate 2 was further reacted, using either Method A or Method B,to give the product 6 in about 70% yield. Yields were found to besomewhat improved by removal of triethylamine (e.g. by aqueous washings)prior to benzoylation, particularly when using Method A. This treatmentwas observed to prevent formation of a side product believed to resultfrom ring opening of the adenine base. Formation of this side productwas also eliminated by isolating the tritylated product 2 before furtherreaction.

C. Method 3: TBDMS Protection Route (as Illustrated in FIG. 1, 1→2→5→6)

The starting monomer, 3′-amino-2′,3′-dideoxyadenosine (1, 10 mmol), wasconverted to the 3′-protected monomer 2 as described above, i.e. bytreatment with 1.1 eq. trityl chloride in dry pyridine containing 6 eq.triethylamine at 50° C.

To the clear solution was added 2 eq. tert-butyldimethylsilyl chloride(TBDMS Cl), and stirring was continued overnight at room temperature.This solution (of the 3′-tritylamino-5′-TBDM silyloxy intermediate 4)was cooled to −5° C., and 3 eq. benzoyl chloride was added dropwise,following by stirring at room temperature for two hours, providing theN⁶-dibenzoyl intermediate 5. The solution was concentrated in vacuo, andthe oily residue was taken up in ethyl acetate, washed with satd. sodiumbicarbonate, dried and evaporated in vacuo.

The oily residue obtained was dissolved in 100 ml THF, and 2 eq. TBAF(tetrabutylammonium fluoride) was added. The mixture was stirredovernight at room temperature. Ethyl acetate was added to the reactionmixture, and the solution was washed with sodium bicarbonate, 0.5 Msodium citrate (pH 4) and brine. The product was purified as above bysilica gel chromatography to give 3.3 g (55.5%) of pure compound 6.

Example 2 Synthesis of N⁴-benzoyl-3′-aminotrityl-2′,3′-dideoxycytosine

The synthesis of N⁴-benzoyl-3′-aminotrityl-2′,3′-dideoxycytosine from3′-amino-2′,3′-dideoxycytosine can be carried out in a similar manner,using any of the above procedures, or in accordance with the method ofExample 4, below.

Example 3 Synthesis ofN²-isobutyryl-3′-aminotrityl-2′,3′-dideoxyguanosine from3′-amino-2′,3′-dideoxyguanosine (FIG. 2A)

The starting material, 3′-amino-2′,3′-dideoxyguanosine (7, 2 g) wascoevaporated with dry pyridine, then suspended in 100 ml dry pyridineand 6 eq. diisopropylethylamine. Tert-butyldimethylsilyl chloride (2eq.) was added, producing a 5′-TBDM silyloxy intermediate. After 30 minstirring, 1.1 eq. of trityl chloride was added in two installments, andthe stirring was continued overnight at room temperature. Two compoundswere detected by TLC, the lower running, more abundant product 8 beingthe desired 5′-TBDMS-3′-aminotrityl-2′,3′-dideoxyguanosine. The higherrunning, less abundant compound 9 is additionally tritylated at thebase. The monotrityl compound 8 was isolated by silica gelchromatography using methylene chloride:methanol:triethylamine 94:5:1v/v/v solvent system.

This product was coevaporated with dry pyridine and dissolved in 100 mlof pyridine. The solution was cooled to 0° C., and 1.1 eq. isobutyrylchloride was added, followed by stirring for 30 min. After quenching thereaction with methanol and evaporation in vacuo, the reaction was workedup from methylene chloride-sodium bicarbonate. The organic phase wasdried over sodium sulfate and evaporated in vacuo. The oily residue (thefully protected monomer 10) was dissolved in 100 ml tetrahydrofuran, and5 eq. tetrabutylammonium fluoride was added. The reaction mixture wasstirred at room temperature overnight, then diluted with ethyl acetate,washed with sodium bicarbonate. After drying over sodium sulfate thesolvent was evaporated in vacuo to give 2 g of the 5′-hydroxyl product11 (46.1%).

In subsequent procedures, tritylation of 7 was carried out inDMF/pyridine at 50° C., and the side product 9 (<5%) was removed byprecipitating the predominant product 8 from dichloromethane or water.The nucleotide base was protected using either the transient protectionscheme of FIG. 2B or a peracylation reaction (analogous to Example 1A).The product 11 was isolated, after crystallization from CH₃CN, in yieldsof about 60% and 53%, respectively. Yields were also observed to improvewhen care was taken to avoid hydrolysis of the acyl chloride reagent.

Example 4 Synthesis of N⁴-benzoyl-3′-aminotrityl-2′,3′-dideoxycytidinefrom 3′-amino-2′,3′-dideoxycytidine (as Illustrated in FIG. 2C)

The 3′-amino group of the starting material 12 was reacted with tritylchloride in 1:4 pyridine:DMF in the presence of triethylamine. Theintermediate 13 was reacted with benzoyl anhydride in 9:1 CH₃CN:MeOH at50° C. The desired product 14 was isolated, following silica gelchromatography, in 70% yield.

Example 5 Synthesis of3′-aminotrityl-3′-deoxy-thymidine-5′-(2-cyanoethyl,N,N-diisopropyl)phosphoramidite from 3′-amino-3′-deoxythymidine,Followed by Phosphitylation (as Illustrated in FIG. 3)

The starting material, 3′-amino-3′-deoxythymidine (15, 1.3 g) wasco-evaporated with dry pyridine, then dissolved in 30 ml dry pyridine.To this solution was added 5 eq. diisopropylethylamine or triethylamine,followed by 10 min stirring and addition of 1 to 1.1 eq. tritylchloride. The reaction mixture was stirred at room temperature overnight(or, alternatively, at 50° C. for 1 hour). After disappearance of thestarting material by TLC, the reaction was quenched with methanol andthe solution evaporated in vacuo. The oil obtained was dissolved inmethylene chloride, washed with satd. sodium bicarbonate, dried oversodium sulfate and concentrated. Methylene chloride-hexane precipitationgave 2.2 g (85%) of the product 16 as a white powder.

The product 16 (1.7 g) was coevaporated with dry pyridine and dissolvedin 100 ml of dry methylene chloride. To this solution was added 4 eq. ofdiisopropylethylamine, followed by 1.2 eq.2-cyanoethyl-N,N-diisopropylchlorophosphoramidite. The reaction wasmonitored by TLC, and after the disappearance of the starting materialthe solution was washed with satd. sodium bicarbonate, dried over sodiumsulfate and evaporated in vacuo. The product was purified by silica gelflash chromatography using methylene chloride:triethylamine 10:1 v/vsolvent system to yield 2 g (83.3%) of the phosphitylated protectedmonomer 17 as a solid foam. The overall yield of the procedure was70.8%.

While the invention has been described with reference to specificmethods and embodiments, it will be appreciated that variousmodifications may be made without departing from the invention.

1. A method of preparing an adenosine, guanosine or cytosine monomerhaving a protected nucleoside base and a protected 3′-amino group,wherein said base and said 3′-amino group are orthogonally protected,the method comprising: (a) providing a 3′-amino-3′-deoxy adenosine,cytosine, or guanosine monomer in which the 5′-hydroxyl group,nucleoside base, and 3′-amino group are unprotected; (b) selectivelyreacting said 3′-amino group with a first protecting group; reactingsaid 5′-hydroxyl group with a second protecting group; and reacting saidnucleoside base with a third protecting group; wherein said firstprotecting group can be removed from said 3′-amino group underconditions which do not deprotect said nucleoside base, and said secondprotecting group can be removed from said 5′-hydroxyl group underconditions which do not deprotect said nucleoside base or said 3′-aminogroup.
 2. The method of claim 1, further comprising the step of (c)removing said second protecting group from said 5′-hydroxyl group, underconditions which do not deprotect said nucleoside base or said 3′-aminogroup.
 3. The method of claim 1, wherein said first protecting group isacid labile.
 4. The method of claim 3, wherein said first protectinggroup is triarylmethyl.
 5. The method of claim 4, wherein said firstprotecting group is triphenylmethyl (trityl).
 6. The method of claim 1,wherein said monomer is an adenosine or cytosine monomer.
 7. The methodof claim 1, wherein said second and third protecting groups are bothacyl groups.
 8. The method of claim 7, wherein said second and thirdprotecting groups are both benzoyl groups.
 9. The method of claim 1,wherein said second protecting group is a base labile trialkyl silylether, and said third protecting group is an acyl group.
 10. The methodof claim 1, wherein said second protecting group is a fluoride labilesilyl ether, and said third protecting group is an acyl group.
 11. Themethod of claim 1, wherein said monomer comprises a 2′ group selectedfrom hydrogen, hydroxy, lower alkoxy, lower alkyl, and fluoro.
 12. Themethod of claim 11, wherein said monomer is a 2′,3′-dideoxy monomer,such that said 2′ group is hydrogen.
 13. A method of preparing anthymidine monomer having a free 5′-hydroxyl group and a protected3′-amino group, the method comprising: (a) providing a 3′-amino-3′-deoxythymidine monomer in which the 5′-hydroxyl group and 3′-amino group areunprotected; and (b) selectively reacting said 3′-amino group with afirst protecting group.
 14. The method of claim 13, wherein said firstprotecting group is acid labile.
 15. The method of claim 14, whereinsaid first protecting group is triarylmethyl.
 16. The method of claim15, wherein said first protecting group is triphenylmethyl (trityl). 17.An adenosine, guanosine or cytosine monomer having a protected 3′-aminogroup and a nucleoside base which is (i) unprotected or (ii) protectedsuch that said protected 3′-amino group can be deprotected underconditions which do not deprotect said nucleoside base.
 18. The monomerof claim 17, wherein said nucleoside base is unprotected.
 19. Themonomer of claim 17, wherein said 3′-amino group is protected with anacid labile protecting group.
 20. The monomer of claim 19, wherein saidacid labile protecting group is a triarylmethyl group.
 21. The monomerof claim 17, wherein said nucleoside base is protected with an acylgroup.
 22. The monomer of claim 17, wherein said nucleoside base isprotected with a dialkylformamidinyl group.
 23. The monomer of claim 17,having an unprotected 5′-hydroxyl group.
 24. The monomer of claim 17,having a protected 5′-hydroxyl group which can be deprotected underconditions which do not deprotect said nucleoside base or said 3′-aminogroup.
 25. The monomer of claim 17, wherein said monomer comprises a 2′group selected from hydrogen, hydroxy, lower alkoxy, lower alkyl, andfluoro.
 26. The monomer of claim 25, wherein said monomer is a2′,3′-dideoxy monomer, such that said 2′ group is hydrogen.
 27. A methodof preparing an adenosine, guanosine or cytosine monomer having a free5′-hydroxyl group, a protected nucleoside base, and a protected 3′-aminogroup, wherein said base and said 3′-amino group are orthogonallyprotected, the method comprising: (a) providing a 3′-amino-3′-deoxyadenosine, cytosine, or guanosine monomer in which the 5′-hydroxylgroup, nucleoside base, and 3′-amino group are unprotected; (b)selectively reacting said 3′-amino group with a first protecting group;and (c) selectively reacting said nucleoside base with a furtherprotecting group; wherein said first protecting group can be removedfrom said 3′-amino group under conditions which do not deprotect saidnucleoside base.
 28. The method of claim 27, wherein said monomer is acytosine monomer.
 29. The method of claim 28, wherein said reacting witha further protecting group comprises reaction with an acyl anhydride.30. The method of claim 27, wherein said monomer is an adenosine orguanosine monomer, and said further protecting group is adialkylformamidinyl, di(cycloalkyl)formamidinyl, or dibenzylformamidinylgroup.
 31. The method of claim 27, wherein said first protecting groupis acid labile.
 32. The method of claim 31, wherein said firstprotecting group is triarylmethyl.